PROGRAMA DE P ´OS-GRADUAC¸ ˜AO EM ENGENHARIA MEC ˆANICA
Eduardo Lucas Konrad Burin
PLANTAS DE COGERAC¸ ˜AO DO SETOR SUCROALCOOLEIRO ASSISTIDAS POR CONCENTRADORES PARAB ´OLICOS
Florian´opolis 2015
PROGRAMA DE P ´OS-GRADUAC¸ ˜AO EM ENGENHARIA MEC ˆANICA
Eduardo Lucas Konrad Burin
PLANTAS DE COGERAC¸ ˜AO DO SETOR SUCROALCOOLEIRO ASSISTIDAS POR CONCENTRADORES PARAB ´OLICOS
Tese submetida ao Programa de P´os-Graduac¸˜ao em Engenharia Mecˆanica da Universidade Federal de Santa Catarina para a obtenc¸˜ao do grau de Doutor em Engenharia Mecˆanica.
Orientador: Prof. Dr. Edson Bazzo
Florian´opolis 2015
Burin, Eduardo Lucas Konrad
Plantas de cogeração do setor sucroalcooleiro assistidas por concentradores parabólicos / Eduardo Lucas Konrad Burin
orientador, Edson Bazzo -Florianópolis, SC, 2015. 157 p.
Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnol6gico. Programa de Pós-Graduação Engenharia Mecânica.
Inclui referências
1. Engenharia Mecânica. 2. Bagaço de cana. 3. Cogeração.
4. Energia termossolar. 5. Hibridização. I. Bazzo, Edson. II. Universidade Federal de Santa Catarina. Programa de Pós Graduação em Engenharia Mecânica. III. Título.
PLANTAS DE COGERAC¸ ˜AO DO SETOR SUCROALCOOLEIRO ASSISTIDAS POR CONCENTRADORES PARAB ´OLICOS
Esta Tese foi julgada adequada para obtenc¸˜ao do T´ıtulo de Doutor em Engenharia Mecˆanica e aprovada em sua forma final pelo Programa de P´os-Graduac¸˜ao em Engenharia Mecˆanica da Universidade Federal de Santa Catarina.
Florian´opolis, 31/07/2015.
Armando Albertazzi Gonc¸alves Jr., Dr. Coordenador do Curso Banca Examinadora:
Edson Bazzo, Dr., Orientador
Silvia Azucena Nebra de P´erez, Dra.
Paulo Smith Schneider, Dr.
Rog´erio Gomes de Oliveira, Dr.
Samuel Luna de Abreu, Dr.
Iloina Konrad.
a arte ´e longa,
a oportunidade ´e fugaz, a experiˆencia enganosa, o julgamento dif´ıcil.
Ao professor Edson Bazzo, meu orientador, pela amizade, orientac¸˜ao e incentivo. Agradec¸o pela confianc¸a depositada em mim durante esta etapa.
`
A minha m˜ae, Mirian, ao meu pai, Neivaldo e `a minha irm˜a, Raquel, pelo carinho, dedicac¸˜ao, educac¸˜ao e apoio em cada passo da minha vida.
A toda minha fam´ılia, pelo apoio ao meu projeto de vida. `
A Glaucia Medeiros, pelo carinho, atenc¸˜ao, amizade e incentivo. Ao Laborat´orio de Combust˜ao e Engenharia de Sistemas T´ermicos (LabCET) da Universidade Federal de Santa Catarina (UFSC) pela oportuni-dade e apoio fundamental em relac¸˜ao ao desenvolvimento desse trabalho e `a minha formac¸˜ao.
Ao Programa de P´os-Graduac¸˜ao em Engenharia Mecˆanica (P´osMEC) da UFSC, bem como a todos os professores, pela minha formac¸˜ao.
Aos todos os amigos do LabCET, em especial aos professores Ed-son Bazzo e Amir Oliveira, bem como aos colegas Amir De Toni, Raphael Miyake, Alvaro Restrepo, Fabio Kleveston, Renzo Figueroa, Nury Garzon, Marcos Oro, Marco Antˆonio, Rafael Zotto, Eduardo Hartmann, Ricardo Hart-mann, Alexandre Schimidt, Leandro Alves e Julian Barrera pela amizade e momentos compartilhados.
Ao LUAT da Universidade de Duisburg-Essen pelo per´ıodo de douto-rado sandu´ıche. Agradec¸o ao professor Klaus Goerner, ao Dr. Oeljeklaus, `a Sra. Hoffmann e aos colegas Tobias Vogel, Andre Thelen e Sven Multhaupt pela amizade, atenc¸˜ao e contribuic¸˜ao t´ecnica.
Ao engenheiro Rodrigo Luis Mello Fonseca pela amizade desde a graduac¸˜ao e pelo importante aux´ılio t´ecnico prestado durante o desenvolvi-mento deste trabalho. Agradec¸o tamb´em por me apresentar o setor sucroal-cooleiro, o que foi de fundamental importˆancia.
`
A TGM turbinas, em especial aos engenheiros Leonardo Buranello e Pedro Lo Giudice, bem como `a Caldema, em especial ao engenheiro Afrˆanio Lopes, pelas contribuic¸˜oes t´ecnicas.
Ao CNPq pelo aux´ılio por meio da linha de financiamento de bolsas de doutorado. `A CAPES pelo financiamento da minha estadia na Alemanha em est´agio de doutorado sandu´ıche. `A CAPES, ao DAAD e ao GIZ pela viabilizac¸˜ao do projeto iNOPA desenvolvido em parceria com a Universidade de Duisburg-Essen.
A todas as pessoas que contribu´ıram para o desenvolvimento desse trabalho.
Um importante aspecto relacionado `a gerac¸˜ao de energia el´etrica por meio do uso de biomassa consiste na garantia da qualidade e da disponi-bilidade do combust´ıvel ao longo do ano. No setor sucroalcooleiro, como exemplo, bagac¸o encontra-se dispon´ıvel principalmente durante o per´ıodo da safra que na regi˜ao Centro-Sul do Brasil acontece entre os meses de Abril e Dezembro. Nesse contexto, o objetivo deste trabalho consiste em avaliar a viabilidade t´ecnica e econˆomica da integrac¸˜ao de concentradores parab´olicos com os ciclos de cogerac¸˜ao do setor sucroalcooleiro como forma de estender a operac¸˜ao destas plantas para o per´ıodo da entressafra. O potencial foi pre-viamente identificado em func¸˜ao do grande n´umero de plantas de cogerac¸˜ao em regi˜ao em que a incidˆencia de irradiac¸˜ao direta normal ´e adequada para a gerac¸˜ao termossolar. Em relac¸˜ao `a abordagem adotada, o primeiro passo consistiu na definic¸˜ao de uma planta de cogerac¸˜ao base com leiaute e carac-ter´ısticas operacionais identificadas em conjunto com fabricantes de equipa-mentos do setor sucroalcooleiro. A planta ´e equipada com dois geradores de vapor com capacidade de 170 t/h de vapor superaquecido com parˆametros de 67 bar e 525 oC. O vapor superaquecido ´e expandido paralelamente em
duas turbinas, sendo uma de contrapress˜ao (BPST) e outra de condensac¸˜ao (CEST). A moagem anual da usina ´e de trˆes milh˜oes de toneladas de cana por safra. Dois leiautes de integrac¸˜ao foram propostos e avaliados, sendo: (a) aquecimento de ´agua de alimentac¸˜ao com energia solar e (b) gerac¸˜ao de vapor saturado com energia solar para posterior superaquecimento em ge-rador de vapor a biomassa. Como resultados importantes, foram identifica-das as caracter´ısticas da operac¸˜ao dos principais componentes da planta de cogerac¸˜ao em condic¸˜ao fora do ponto de projeto. Foi identificado o potencial de economia de bagac¸o para ambos os casos, bem como o custo nivelado da eletricidade gerada (LCOE) em func¸˜ao da operac¸˜ao h´ıbrida.
Palavras-chave: Cana-de-ac¸´ucar, bagac¸o, cogerac¸˜ao, energia termossolar, concentradores parab´olicos.
One important problem related to the biomass power plants operation consists on the fuel availability along the year. This is also true for the su-garcane bagasse power plants in Brazil that are operated mainly during the sugarcane harvest period that ranges from April to December in the Center-South region. In this regard, the objective of this work was to evaluate the technical and economic feasibility of integrating Concentrated Solar Power (CSP) with the conventional sugarcane bagasse cogeneration power plants in the sugarcane sector in Brazil in order to extend their operation to the off-season period and, as a consequence, to improve the electricity produc-tion. The potential for CSP hybridization with bagasse was identified once both energy sources matches regionally in their availability. Regarding the adopted approach, the first step consisted on the identification of a base case sugarcane bagasse cogeneration power plant, whereby layout and operational parameters were defined in cooperation with equipment suppliers. The coge-neration cycle has two 170 t/h capacity steam generators that provide steam at 67 bar and 525oC. Main steam is expanded in parallel in a backpressure
(BPST) and a condensing-extraction (CEST) steam turbine. Three million tons of sugarcane are processed per harvest. Two integration layouts of pa-rabolic trough concentrators into cogeneration cycle were evaluated, namely: (a) solar feedwater heating; (b) saturated steam generation with solar energy and post superheating in biomass steam generators. As main results, the off-design operation of solar aided plant concepts was here identified considering minimal modifications on the existing infrastructure. The bagasse economy potential due to hybridization as well as the Levelized Cost of Electricity (LCOE) of additional electricity produced was identified and compared with the current commercial CSP plants.
Keywords: Sugarcane, bagasse, cogeneration, concentrated solar power, pa-rabolic trough.
1 World map of DNI. . . 37
2 CSP technologies. . . 38
3 CSP projects around the world. . . 40
4 Capital expenditures (CAPEX) of CSP and PV for comparison. 42 5 Levelized Cost of Electricity (LCOE) of CSP and PV for comparison. . . 43
6 Sugarcane plantation area in Brazil and annual crushing evo-lution. . . 48
7 Evolution of sugar and alcohol (total) production in Brazil. . . . 49
8 Crushed sugarcane according to industries capacity. . . 49
9 Layouts of cogeneration cycles applied to the sugarcane sector. 51 10 Evolution of main steam parameters in the sugarcane sector. . . 53
11 Evolution exported electricity to the grid. . . 53
12 Identified potential of CSP hybridization in the sugarcane sector: a) bagasse power plants location; b) integrated annual DNI potential in Brazil. . . 54
13 Schematic of the process and concept illustration. . . 55
14 Identified integration layouts of CSP and bagasse cogenera-tion cycles to be studied. . . 57
15 Schematic of a parabolic trough solar field. . . 59
16 Schematic of a parabolic trough loop. . . 60
17 Schematic of a HCE (not shown in scale). . . 60
18 SkyTrough parabolic trough collectors. . . 61
19 Incidence angle formed by the vector normal to collector’s aperture area and direct irradiance. . . 63
20 Intercept factor calculation in a parabolic trough concentrator using photogrammetry. . . 65
21 Representation of irradiation end losses. . . 66
22 Illustration of a boiler implemented in a parabolic trough plant. 72 23 Solar field operation stragegy. . . 74
24 Deaerator simplified scheme. . . 79
25 Base case cogeneration power plant layout and simulation re-sults at design point operation. . . 82
26 Base case steam generator simulation results at design point operation. . . 85
performed regarding the unburned carbon content in ash. . . 87 29 Participation of the distinct losses (LHV basis) at part load. . . 88 30 Steam temperature profiles at part load. . . 88 31 Base case cogeneration power plant simulation results at
off-season operation. . . 89 32 Bagasse consumption profile during harvest operation. . . 91 33 Solar-aided (solar feedwater heating) cogeneration plant
si-mulation results at design point operation. . . 96 34 CEST turbine operation results for design point peak
sum-mer weather condition under: a) base case harvest operation; b) off-season operation and c) solar-aided harvest operation (16.7 MW solar thermal load). . . 98 35 Economized bagasse during harvest operating days. . . 99 36 Solar-aided bagasse steam generator simulation results at
de-sign point operation. . . 101 37 Solar field (saturated steam generation) simulation results at
design point operation. . . 101 38 Influence of steam generator’s part load efficiency on solar
electricity LCOE. . . 104 39 Influence of economic assumptions on solar electricity LCOE. 105 40 Solar equivalent electricity and LCOE for different solar
mul-tiples. . . 106 41 Influence DNI and operation period on solar electricity LCOE. 107 42 Comparison of LCOE [2014 U$/MWh] for distinct
technolo-gies. . . 108 43 SEGS VI power plant layout. . . 123 44 Schematic of solar field model verification. . . 124 45 DNI data of two typical operation days used in verification
process presented under two time steps. . . 124 46 Comparison of measured and calculated HTF temperature in
expansion tank outlet (12-minutes time resolution) for two typical operation days. . . 125 47 Comparison of measured and calculated HTF temperature in
expansion tank outlet (1-hour time resolution) for two typical operation days. . . 126 48 Energy balance of steam generators. . . 128 49 CO emission in mg/Nm3as a function of combustion
51 Direct normal solar irradiance monthly 95 % percentiles. . . 146 52 Wind velocity monthly 95 % percentiles. . . 146 53 Ambient temperature (DB) 5 to 95 % percentiles. . . 147 54 DB and WB monthly 95 % percentiles for off-season months. 147 55 Solar-aided (after ECO integration) bagasse steam generator
simulation results at design point operation. . . 149 56 Solar field (after ECO integration) simulation results at
de-sign point operation. . . 150 57 Additional solar equivalent electricity generated. . . 154 58 Thermal energy transferred to water-steam cycle related to
bagasse and solar energy inputs for base case and hybrid layouts 2 and 3 for comparison. . . 154 59 Thermal energy transferred to water-steam cycle related to
bagasse and solar energy inputs for base case and hybrid layouts 2 and 3 for comparison. . . 155 60 a) Investment and O&M costs of solar hybridization; b)
1 Overview to the three main CSP technologies. . . 39 2 Literature data related to CSP hybridization selected for
cross-comparison. . . 46 3 Hybrid CSP projects around the world. . . 47 4 Elemental, proximate and calorimetric (heating value)
analy-sis of sugarcane bagasse and straw. . . 50 5 Coeficients of heat loss model (Equation 4.14) fitted for Luz
Cermet HCE. . . 68 6 Example of commercial models of parabolic trough collectors. 74 7 Cogeneration power plant site and TMY data. . . 81 8 Design point assumptions adopted for cogeneration cycle
si-mulation. . . 83 9 Configuration of heat exchangers used in superheaters,
eco-nomizer and air heaters of studied steam generators. . . 84 10 Design point assumptions adopted for steam generators
simu-lation. . . 84 11 Bagasse mass flow rate and electric power production at
de-sign point operation. . . 85 12 Bagasse mass flow rate and electric power production for
off-season operation at peak summer weather condition. . . 90 13 Base case cycle managed bagasse amount and electricity
ge-neration during harvest period. . . 91 14 General assumptions adopted for solar field sizing and
simu-lation. . . 94 15 Assumptions adopted for economic analysis. . . 95 16 Solar field (solar feedwater heating) results at design point
operation. . . 97 17 Results related to solar aided power plant (solar feedwater
heating). . . 99 18 Solar field (saturated steam generation) simulation results at
design point operation. . . 102 19 Results related to solar aided power plant (saturated steam
generation). . . 103 20 Comparison of solar field energy output calculated by using
measured data and simulated data under distinct time resolu-tions. . . 126 21 Coefficients for dry residue enthalpy calculation. . . 133
use sugarcane bagasse external combustion. . . 134 23 References of thermodynamic and transport properties of air
and flue gas main components. . . 143 24 Solar field (after ECO integration) results at design point
ope-ration. . . 151 25 Results related to solar aided power plant (after ECO
integra-tion). . . 151 26 Number of simulations performed for each layout. . . 153
BPST CAPES CAPEX CEST CF CSP CT DAAD DB DMS DNI DSG EES HCE HHV HSG HTF IAM iNOPA ISCC LabCET LCOE LF LHV LUAT MENA NREL PE1 PT PV SAFWH SAM SCA SEGS SGT SM
Back-Pressure Steam Turbines
Coordination for the Improvement of Higher Education Personnel
Capital Expenditures
Condensing-Extraction Steam Turbines Capacity Factor
Concentrated Solar Power Central Tower
German Academic Exchange Service Dry Bulb
Direct Molten Salts Direct Normal Irradiation Direct Steam Generation Engineering Equation Solverr Heat Collection Element Higher Heat Value
Hybridized Steam Generators Heat Transfer Fluid
Incidence Angle Modifier New Partnerships Program Integrated Solar Combined Cycles
Laboratory of Combustion and Thermal Systems Engineering Levelized Cost of Electricity
Linear Fresnel Lower Heat Value
Chair of Environmental Process Engineering and Plant Design Middle East and North Africa
National Renewable Energy Laboratory Puerto Errado One
Parabolic Trough Photovoltaics
Solar Aided Feedwater Heating System Advisor Model Solar Collector Assembly Solar Energy Generating Systems Solar-Gas Turbines
UFSC Federal University of Santa Catarina
𝐴 Area [m²]
𝐴𝐸 Additional electricity generated [MWh]
𝐴𝑖𝑗 Interaction parameter of gas-mixture conductivity [-]
𝑎𝑒 Air excess [%]
𝐶 Carbon mass fraction [kg/kg]
𝐶𝐶 Capital cost [U$]
𝑐 Mass fraction [kg/kg]
𝑐𝑝 Specific heat at constant pressure [kJ/kg.K]
𝑑 Spacing between rows; Diameter [m; mm]
𝐸̇ Energy transfer rate [kW]
𝐸𝐵 Economized Bagasse [t]
𝐸𝑇 Equation of time [min]
𝑒 Energy transfer per kilogram of burned fuel [kJ/kg]
𝑓 Degradation factor [-]
𝑓𝑙 Average focal length [m]
𝐺𝑏𝑛 Direct normal irradiance [W/m²]
𝐻 Hydrogen mass fraction [kg/kg]
ℎ Enthalpy [kJ/kg]
ℎ𝑐𝑜𝑛𝑣 Convection heat transfer coefficient [kW/m².K]
ℎ𝑟𝑎𝑑 Radiation heat transfer coefficient [kW/m².K]
𝑘 Thermal conductivity [W/m.K]
𝑘Δ𝑝 Pressure drop constant [bar.s²/m²]
𝐿 Length [m]
𝐿𝐶 Land cost [U$]
𝑙𝑡 Life time [years]
𝑀 Mass [kg]
𝑚 Mass flow per kilogram of burned fual [kg/kg]
𝑚̇ Mass flow rate [kg/s]
𝑁 Nitrogen mass fraction [kg/kg]
𝑁𝑢 Nusselt number [-]
𝑛 Day of the year [-]
𝑂 Oxygen mass fraction [kg/kg]
𝑂&𝑀 Operation and maintenance cost [U$]
𝑃𝑟 Prandtl number [-]
𝑝 Pressure [bar]
𝑄̇ Heat transfer rate [W]
𝑄̇′ Heat transfer rate per unit length [W/m]
𝑄̇′′ Heat flux [W/m²]
𝑞𝑝𝑟𝑜 Heat demand per ton of processed sugarcane [kWh/t]
𝑅 Mass of residue per kilogram of burned fuel [kg/kg]
𝑟 Interest rate [%]
𝑆 Sulfur mass fraction [kg/kg]
𝑠 Spacing of tubes [mm]
𝑡 Time; Thickness [h; mm]
𝑡𝑓 Thickness of flame [m]
𝑈𝐴 Overall heat transfer coefficient-area product [kW/K]
𝑉 Velocity [m/s]
𝑉̇ Volume flow rate [m³/s]
𝑣 Volume flow per kilogram of burned fuel [Nm³/kg]
𝑊̇ Rate at which work is performed [W]
𝑤 Width [m]
𝑤𝑝𝑟𝑜 Electricity demand per ton of processed sugarcane [kWh/t]
𝑥 Quality [-]
𝑦 Mole fraction [kmol/kmol]
Greek letters
𝛼 Absorptivity; Ash collection point [-]
𝛼𝑠 Solar altitude angle [rad]
𝛿 Declination angle [rad]
𝜀∞ Emissivity of a very tick flame [-]
𝜀𝑓𝑠 Effective emissivity between flame and surface [-]
𝜂 Efficiency [%]
𝜃 Incidence angle [rad]
𝜃𝑧 Zenith angle [rad]
𝜇 Dynamic viscosity [Pa.s]
𝜈 Kinematic viscosity [m²/s]
𝜌 Reflectivity; Mass density [-; kg/m³]
𝜏 Transmissivity [-]
𝜙 Latitude angle [deg]
𝜙𝑖𝑗 Interaction parameter of gas-mixture viscosity [-]
𝜒 Longitude angle [deg]
𝜓 Azimuth angle [rad]
𝜔 Time angle [rad]
𝜔𝑎𝑖𝑟 Air humidity ratio [kg/kg]
Subscripts and superscripts
𝑎𝑏𝑠 Absorbed
𝑎𝑑 Adiabatic
𝑎𝑑𝑑 Additional
𝑏 Bagasse 𝑏𝑐 Base case 𝑏𝑑 Blowdown 𝑏𝑠 Bleed-off steam 𝐶 Carbon 𝑐𝑑 Condenser 𝑐𝑟𝑒𝑑 Credits 𝑐𝑡 Cooling tower
𝑑 Dry; Diameter; Diagonal
𝑑𝑚 Direct method 𝑒 Effective 𝑒𝑏 Energy balance 𝑒𝑐𝑜 Economizer 𝑒𝑠 Exhaust steam 𝑒𝑣𝑎𝑝 Evaporator 𝑒𝑥𝑡 External 𝑓 Flame 𝑓𝑒 Furnace exit 𝑓𝑔 Flue gas 𝑓𝑤 Feedwater 𝑔 Gross ℎ Hybrid ℎ𝑑 Header pipes
ℎ𝑓𝑤 High pressure feedwater heater ℎ𝑡𝑓 Heat transfer fluid
𝑖𝑓 Intercept factor
𝑖𝑛 Inlet
𝑖𝑛𝑡 Internal
𝑙 Longitudinal; Loss
𝑙𝑓𝑤 Low pressure feedwater heater
𝑙𝑜𝑐 Local 𝑚 Mixture 𝑚𝑎𝑥 Maximum 𝑚𝑖𝑛 Minimum 𝑜𝑢𝑡 Outlet 𝑝 Pump 𝑝𝑟𝑜 Process 𝑟 Residue 𝑟𝑒𝑓 Reference 𝑠ℎ Shadowing; Superheated 𝑠𝑐 Sugarcane 𝑠𝑒 Solar-to-electricity 𝑠𝑓 Solar field
𝑡 Tank; Transversal; Turbine 𝑡𝑟𝑘 Tracking system
𝑢𝑛𝑏 Unburned
1 INTRODUCTION . . . 33 1.1 SCOPE OF PROPOSAL . . . 33 1.2 GOALS . . . 34 1.2.1 Main goal . . . 34 1.2.2 Specific goals . . . 34 1.3 STRUCTURE OF DOCUMENT . . . 35 2 BACKGROUND. . . 37 2.1 CONCENTRATED SOLAR POWER . . . 37 2.1.1 Solar resources around the world . . . 37 2.1.2 CSP technologies . . . 38 2.1.3 The current world scenario . . . 40 2.1.4 Technology costs . . . 41 2.1.5 CSP hybridization . . . 43 2.1.6 Projects around the world . . . 47 2.2 THE BRAZILIAN SUGARCANE SECTOR . . . 48 2.2.1 Sugarcane sector figures . . . 48 2.2.2 Cogeneration cycles . . . 50 2.2.3 The identified potential . . . 53 3 HYBRID CONCEPT PROPOSAL . . . 55 3.1 OPERATION STRATEGY . . . 55 3.2 SELECTED INTEGRATION LAYOUTS . . . 55 3.3 THERMOECONOMIC ANALYSIS . . . 56 4 SOLAR FIELD MODELING. . . 59 4.1 MAIN COMPONENTS . . . 59 4.2 GEOGRAPHICAL AND METEOROLOGICAL DATA . . . 61 4.3 LOCAL AND SOLAR TIME . . . 62 4.4 INCIDENCE ANGLE . . . 62 4.5 SOLAR IRRADIANCE ABSORPTION . . . 63 4.5.1 Peak optical efficiency . . . 64 4.5.2 Incidence angle modifier . . . 65 4.5.3 Shadowing and end losses . . . 65 4.5.4 Additional factors . . . 66 4.6 HEAT LOSSES . . . 67 4.6.1 Heat collector elements . . . 67 4.6.2 Header pipes . . . 68 4.7 NET HEAT RATE AND TEMPERATURE OUTPUT . . . 69 4.8 THERMAL INERTIA . . . 69
4.10 HEAT EXCHANGERS MODELING . . . 71 4.10.1 Economizer . . . 71 4.10.2 Boiler . . . 71 4.11 OPERATION STRATEGY . . . 72 4.12 COMMERCIAL COMPONENTS . . . 73 4.13 MODEL VERIFICATION . . . 73 5 COGENERATION PLANT MODELING . . . 75 5.1 STEAM GENERATORS . . . 75 5.2 STEAM TURBINES . . . 75 5.2.1 Steam expansion across a turbine stage . . . 75 5.2.2 Efficiency at off-design operation . . . 76 5.2.3 Power output . . . 76 5.3 CONDENSER AND COOLING TOWER . . . 77 5.4 PUMPS . . . 77 5.5 FEEDWATER HEATERS . . . 78 5.6 DEAERATOR . . . 78 5.7 HEAT AND ELECTRICITY PROCESS DEMAND . . . 79 5.8 HARVEST OPERATING DAYS . . . 80 6 BASE CASE COGENERATION PLANT. . . 81 6.1 GENERAL ASSUMPTIONS . . . 81 6.2 COGENERATION PLANT DESCRIPTION . . . 81 6.3 OFF-DESIGN SIMULATION . . . 85 6.4 HARVEST SIMULATION . . . 90 7 HYBRID LAYOUTS . . . 93 7.1 GENERAL ASSUMPTIONS . . . 93 7.2 RESULTS FOR SOLAR FEEDWATER HEATING . . . 95 7.2.1 Solar integration at design point condition . . . 95 7.2.2 Annual analysis . . . 98 7.3 RESULTS FOR SOLAR SATURATED STEAM GENERATION 100 7.3.1 Solar integration at design point condition . . . 100 7.3.2 Annual analysis . . . 102 7.4 SENSITIVE ANALYSIS . . . 103 7.4.1 Part load efficiency of steam generators . . . 103 7.4.2 Economic assumptions . . . 104 7.4.3 Solar multiple sensitive analysis . . . 105 7.4.4 DNI incidence and operation period . . . 107 7.5 SUMMARY AND OUTLOOK . . . 108 8 CONCLUSIONS. . . 111
Appendix A – Solar field model verification. . . 123 Appendix B – Steam generators modeling . . . 127 B.1 ENERGY BALANCE . . . 127 B.1.1 Combustion . . . 129 B.1.2 Energy credits . . . 131 B.1.2.1 Entering air . . . 131 B.1.2.2 Sensible heat in fuel . . . 131 B.1.3 Energy losses . . . 131 B.1.3.1 Flue gas . . . 131 B.1.3.2 Dry residue sensible heat . . . 132 B.1.3.3 Unburned carbon . . . 133 B.1.3.4 Hydrocarbon emissions . . . 133 B.1.3.5 Heat loss to ambient . . . 134 B.1.4 Energy output . . . 135 B.1.5 Efficiency calculation . . . 136 B.2 HEAT TRANSFER ANALYSIS . . . 137 B.2.1 Combustion chamber . . . 137 B.2.2 Heat exchange in bundle of tubes . . . 139 B.2.3 Internal convection in tubes . . . 140 B.2.4 External convection in bundle of tubes . . . 140 B.2.5 Gas radiation in bundle of tubes . . . 142 B.3 THERMOPHYSICAL PROPERTIES OF SUBSTANCES . . . 143 B.3.1 Steam properties . . . 143 B.3.2 Air and flue gas elements . . . 143 B.3.3 Properties of gas mixtures . . . 143 B.4 DYNAMIC SIMULATION OF STEAM GENERATORS . . . 144 Appendix C – Design point weather conditions . . . 145 C.1 SOLAR FIELD PARAMETERS . . . 145 C.2 COOLING TOWER PARAMETERS . . . 146 Appendix D – Solar saturated steam generation: an alternative
design. . . 149 D.1 DESIGN POINT . . . 149 D.2 ANNUAL ANALYSIS . . . 151 Appendix E – Summary of iNOPA project results. . . 153 Annex A – Heat transfer fluid properties. . . 157
1 INTRODUCTION
1.1 SCOPE OF PROPOSAL
Solar and biomass are both renewable energy resources which contri-bute to the electricity generation at low CO2emission levels. One important
problem related to the biomass power plants operation, however, consists on its availability along the year. This is also true for the sugarcane bagasse power plants in Brazil that are operated mainly during the sugarcane harvest period. In this regard the Concentrated Solar Power (CSP) hybridization of biomass plants has been studied under different configurations. The central idea consists on displacing fuel consumption during sunny hours and provi-ding power supply on a biomass only mode during hours of no solar irradi-ation incidence - the so-called fuel economy hybridizirradi-ation mode. Solar heat load can also be used to provide power boost during sunny hours - the so-called power boost hybridization mode. These approaches can be applied to new plants and also on existing ones by performing the retrofit of compo-nents. Sharing common infrastructure turns possible the reduction of solar energy implementation costs.
The installed capacity of sugarcane bagasse cogeneration plants in Brazil reached 9,930 MW in the first semester of 2015. This amount is pro-duced by 387 units and represents 6.9 % of the Brazilian electricity installed capacity (ANEEL, 2015). In the last decade it has started the modernization cycle of these units aiming the increase of power exportation to the grid. This was motivated by the Brazilian electricity sector decentralization in 2000. Since then, academic works have also been developed to increase these indi-cators as electricity today consists on an additional product beyond sugar and alcohol (ALVES, 2011;SEABRA, 2008;NETO; RAMON, 2002). In 2013 a total of 15,067 GWh of electricity generated by the sugarcane bagasse cogeneration plants was exported to independent consumers supplying around 8 million homes. Since 2005, an average yearly growth in the electricity exportation of 34 % has been observed (SOUZA, 2014).
The cogeneration plants of sugarcane sector are fueled with bagasse which is a residue obtained after the juice extraction process out of sugarcane culms. The operation takes place during the sugarcane harvest that extends from April to December in the center-south region of Brazil (BNDES; CGEE, 2008). In the rest of the year, most plants remain out of operation and no electricity is produced. In this regard, it was identified in Brazil an
opportu-nity related to the integration of solar thermal energy with the cogeneration plants of sugarcane sector. The solar integration in a fuel economy mode du-ring harvest might provide bagasse reserves which can be used to operate the power plants during off-season. This might minimize the seasonality effect inherent to this crop.
No preliminary works were identified in literature up to now related to the hybridization of cogeneration plants of sugarcane sector with CSP in order to increase electricity exportation to the grid. Thus, in this work this concept is presented and a case study is performed in order to evaluate the integration of a parabolic trough solar field with a typical sugarcane bagasse plant located in Campo Grande, in the State of Mato Grosso do Sul, under thermodynamic and economic aspects. Two integration concepts were evaluated, namely: (a) solar feedwater heating; (b) saturated steam generation with solar energy and post superheating in biomass steam generators. The scope was here limited to the retrofit of conventional cogeneration plants aiming minimal modifications on original installations.
1.2 GOALS
1.2.1 Main goal
The main goal of this work consists in improving the electricity expor-tation capacity of existing cogeneration power plants applied to the sugarcane sector by integrating them with parabolic trough collectors.
1.2.2 Specific goals
The specific goals are described below:
• Develop a simulation model to reproduce the operation and perfor-mance of a parabolic trough solar field;
• Develop a simulation model to reproduce the operation and perfor-mance of bagasse cogeneration cycles;
• Define a base case scenario cogeneration plant based on contacts per-formed with equipment suppliers and sugar and alcohol producers of the sugarcane sector;
• Design and evaluation of distinct integration layouts between the base case cogeneration plant and a parabolic trough solar field in a retrofit scenario;
• Define thermodynamic and economic performance indexes to evaluate the integration layouts;
• Propose an evaluation method to perform additional case studies of CSP hybridization with sugarcane bagasse cogeneration plants.
1.3 STRUCTURE OF DOCUMENT
The literature review on CSP electricity generation and on the Brazi-lian sugarcane sector is presented on Chapter 2 in order to provide the initial contextualization.
In Chapter 3 it is presented the proposed hybridization method of ba-gasse cogeneration cycles with CSP, the identified integration layouts and the thermodynamic and economic performance indexes.
The parabolic trough simulation models are presented in Chapter 4, while the implemented models to perform the simulation of bagasse cogene-ration cycles are described in Chapter 5.
The base case cogeneration power plant description and simulation results are presented in Chapter 6.
The hybridized cogeneration power plant is described in Chapter 7. In this chapter the results related to the identified integration layouts of cogene-ration plant with parabolic trough solar field are compared.
2 BACKGROUND
A literature review on Concentrated Solar Power (CSP) electricity ge-neration (Section 2.1) and on the Brazilian sugarcane sector (Section 2.2) is presented in this chapter. The presented information was considered in order to define precisely the scope of this work.
2.1 CONCENTRATED SOLAR POWER
2.1.1 Solar resources around the world
The operation of CSP plants depends on the incidence of Direct Nor-mal Irradiation (DNI). The annual DNI incidence around the world is presen-ted in Figure 1. The regions with the greatest DNI are the deserts of Middle East and North Africa (MENA), the South Africa, the North-Western India, the Southern of the United States, Mexico, Peru, Chile, Northeast of Bra-zil, Australia and Southern Spain. The accumulated DNI in the South of Spain can reach 2,100 kWh/m2-year, while it can reach 3,600 kWh/m2-year in
North Chile. In South-West of United States, the DNI index of 2700 kWh/m2
-year is reached. In Brazil, the DNI can overcome 2100 kWh/m2-year in the region of S˜ao Francisco river basin.
Figure 1: World map of DNI.
Source: (SOLARGIS, 2014).
economic feasibility depends on specific characteristics of the project under evaluation, like the following aspects: capital and O&M costs, capacity factor and efficiency of plant, the use of hybridization, electricity contract price, interest rate, among other.
2.1.2 CSP technologies
In general, the ways of concentrating solar energy can be divided into linear and point focusing. Parabolic trough and linear Fresnel belong to the linear focusing technologies, whereas the point focusing are namely central tower and parabolic dish reflectors - see Figure 2. The technical characteris-tics of the three main commercial CSP technologies are presented in Table 1 for comparison1.
Figure 2: CSP technologies.
Sources: (SHAMS POWER COMPANY PJSC, 2015); (AREVA SOLAR, 2015); (TORRESOL ENERGY, 2015); (PLATAFORMA SOLAR DE ALMERIA (PSA), 2015)
Parabolic trough represents today the most mature CSP technology, with 88 % share in terms of installed capacity around the world. Typically,
1The parabolic dish system is out of scope here once its application is related to distributed generation on a small scale (on the order of some kW).
Table 1: Overview to the three main CSP technologies.
Parameter Unity Fresnel Trough Tower
Heat transfer fluid - thermal oil thermal oil water
(HTF) water water molten salt
molten salt air
Concentration ratioa - >60 70-80 >1,000 Temperature rangea oC 250-500 350-550 565-1,000* Output rangea MW 10-200 10-300 10-200 Peak ηsea,b,∗∗ % 18 14-24 23-35 Annual ηsea,∗∗ % 9-13 11-16 7-20 Capacity factora % 22-24 25-28 63 (15 h) 43 (7 h) Capacity ratioc % 1 88 11 Construction ratiod % 6 75 18
*Gas turbine applications; **Solar-to-electricity efficiency; Sources:a(IRENA, 2012); b(GIOSTRI et al., 2012);c(SOLARPACES, 2015);d(RENEWABLES. . ., 2013).
this system is operated with thermal oil as heat transfer fluid (HTF) with its temperature limited to around 400 oC. The use of molten salts and direct steam generation (DSG) in trough collectors is currently under development. The capacity factor (CF) of parabolic trough power plants without thermal storage ranges from 25 to 28 %, depending mainly on annual DNI incidence level. The storage of hot thermal oil during sunny hours, nevertheless, pro-vide the improvement of CF to around 40 % - representing 7 h operation at turbine’s design point full load. See Turchi (2010) for the description of a typical parabolic trough plant design with thermal storage system.
The linear Fresnel technology emulates the parabolic trough collectors by using a set parallel rows of flat, or slightly curved, glass mirrors to focus direct solar irradiation onto a linear receiver. Once receiver doesn’t move as system tracks the sun position, it is suitable for high pressure DSG. An addi-tional special characteristic of Fresnel collectors consists on the compactness of solar field in terms of land usage. The distance between rows in loops can be reduced from 12.5-18 m (trough collectors) up to around 4.5 m. Thermal storage in DSG systems is today in stage of development and typically limi-ted to around 1 h operation at turbine’s full load. Tests are being performed in Spain in the Direct Molten Salts (DMS) demonstration project at Puerto Errado One plant (PE1) to adapt Fresnel collectors to the use of molten salts in order to improve heat storage capacity (NOVATEC SOLAR, 2015). As a draw-back, the optical efficiency of Fresnel collectors is lower in comparison with parabolic trough especially in the beginning and in the end of the days when
sun is close to horizon, as it is stated by Morin et al. (2012).
The central tower point focusing technology can provide the DNI con-centration ratio to above 1,000 and thus it is feasible to reach as high as 1,000oC operating temperatures. Central tower systems can be operated with DSG, molten salts or yet with air as HTF. The use of molten salts is normally associated with thermal storage. Due to the higher operating temperature in comparison with trough systems, the CF of central tower plants can reach up to 63 % - representing 15 h operation at turbine’s full load. This is true once the higher the temperature difference in terms of hot and cold storage tanks, the smaller the necessary HTF volume to provide heat storage and, as a con-sequence, investment costs are reduced. Finally, due to the higher operating temperatures, the annual solar-to-electricity efficiency of tower systems tend to be higher in comparison with trough and Fresnel plants.
2.1.3 The current world scenario
The distribution of CSP electricity capacity around the world under the status of operational, under construction and under development is presented in Figure 3. Currently the installed capacity of CSP power plants is around 4 GW. Spain represents 58 % of CSP installed capacity, followed by USA that represents around 39 %.
Figure 3: CSP projects around the world.
Source: (SOLARPACES, 2015).
In Spain, the improvement of the CSP installed capacity was reached due to governmental incentives which provided feasible and stable prices for renewable electricity commercialization. The Electricity Industry Act in 1997
created the Special Regime, in which it was grouped renewable power gene-ration and cogenegene-ration plants (except the hydropower unities). Incentives were fully guaranteed to CSP plants limited to 50 MW capacity: they consis-ted in a fixed charge for the participation in the controlled electricity market, or the market price added by a fixed supplement in case of participation in the open electricity market (BOE, 2011). Today, due to economic aspects, the incentives for CSP were ended. This is in agreement with the stagnation in terms of new projects in Spain.
The development of CSP market began in the eighties in USA with the Solar Energy Generating Systems (SEGS) parabolic trough power plants - 354 MW. After 20 years of absence of new projects due to the reduced oil prices, the improvement of CSP capacity started again in 2007 in Spain, later in USA (2012) and currently in Chile, MENA region, South Africa and China. Despite the recent growth, CSP market is yet in its infancy in compari-son with other renewable electricity generation technologies. As an example, the installed capacity of photovoltaics today is around 139 GW, from which around 80 GW is installed in Europe (EPIA, 2014).
2.1.4 Technology costs
Ranges for CSP capital expenditure costs (CAPEX) under different solar field technologies and energy storage capacities are summarized in Fi-gure 4. The CAPEX range for ground mounted photovoltaic plants (PV) is also presented for comparison. The linear Fresnel (LF) system has the pur-pose of presenting reduced investment costs due to the simpler solar field array. In both parabolic trough (PT) and central tower (CT) technologies, the use of energy storage is related to a significant increment in CAPEX. Ne-vertheless, note that the cost of a trough plant with 6 h storage is compatible with a central tower plant with 12-15 h storage capacity. As exposed in Sec-tion 2.1.2, the higher temperature difference in terms of hot and cold tanks turns possible a smaller HTF volume to provide heat storage and, as a con-sequence, the investment costs are reduced. Data related to photovoltaics are for the ground mounted higher capacity systems, which are more comparable with CSP plants. Note that these systems have lower CAPEX in comparison with any CSP configuration here presented.
Ranges of Levelized Cost of Electricity (LCOE) for parabolic trough2
2Linear Fresnel LCOE costs are not reported due to reduced references currently under ope-ration.
Figure 4: Capital expenditures (CAPEX) of CSP and PV for comparison.
Sources: (IRENA, 2012); (KOST et al., 2013).
and central tower technologies under different storage capacities and for ground mounted photovoltaic plants are summarized in Figure 5. The LCOE method allows the electricity generating costs comparisson of different power plants. The sum of all accumulated costs for building and operating a plant during its life time is compared with the sum of electricity power generation. This then yields the electricity cost necessary to break-even the investment. It is important to note that LCOE cannot be directly considered to define if an investment is feasible or not. For that, a detailed financing calculation must be completed (KOST et al., 2013;IEA, 2010).
The LCOE depends primarily on capital costs, capacity factor (which is related to local DNI incidence and storage capacity) and interest rate. The CSP LCOE data reported by IRENA (2012) is based on a 10 % interest rate, while the photovoltaics LCOE data reported by Kost et al. (2013) is based on a range of 4-6 % interest rate values. The CSP energy projects are currently considered more risky by financiers due to their less mature technology status and limited amount of references around the world.
Despite the higher CAPEX of CSP systems equipped with energy sto-rage, the LCOE tends to be reduced due to the improvement of capacity factor. Parabolic trough plants without storage have LCOE of 300-370 U$/MWh, while it can be reduced as low as 210 U$/MWh if a 6 h storage system is used. The LCOE related to central tower technology ranges from 170 to 240 U$/MWh if a 12-15 h storage system is used.
The LCOE of ground mounted photovoltaic utilities (80-182 U$/MWh) is lower in comparison with any CSP configuration here presented. Neverthe-less, the main advantage of CSP is the possibility of energy storage. A storage
Figure 5: Levelized Cost of Electricity (LCOE) of CSP and PV for comparison.
Sources: (IRENA, 2012); (KOST et al., 2013).
system can be implemented in order to shift the generation of electricity for the early evening when there is a peak in demand. Today it is economically unfeasible to store electricity after its generation in photovoltaic panels. In addition, as it will be discussed in Section 2.1.5, CSP is an interesting option for hybridization with traditional fuels like, coal, natural gas or biomass due to similarity by using a water/steam cycle in order to transform the heat energy in electricity. This concept may provide the possibility of firm electricity generation independent on DNI incidence regularity.
Finally, according to IRENA (2012), significant cost reductions in CSP are expected. They might come from economies of scale, learning ef-fects, advances in R&D, a more competitive supply chain and improvements in the performance of the solar field, solar-to-electric efficiency and thermal energy storage systems. Capital cost reductions of 28% to 40% are expected up to 2020, what will bring a direct improvement in LCOE.
2.1.5 CSP hybridization
The CSP hybridization can not only be applied to new plants but also to existing ones. Sharing common infrastructure turns possible the reduction of solar energy generation costs. Furthermore, if solar energy is used to dis-place fuel consumption during sunny hours, there is the possibility of base load power supply without the implementation of thermal storage systems.
Solar energy can be integrated into combined cycles - Integrated So-lar Combined Cycles (ISCC). In ISCC, soSo-lar thermal load is used to produce
steam to displace fuel consumption or to generate additional power. Solar-to-electricity efficiency greater than for equivalent operating temperature solar-only plants can be achieved at reduced CAPEX despite the necessity of re-sized steam turbine in the bottoming cycle (ZHU et al., 2015; MONTES et al., 2011). Compressed air receivers represent a technology currently under de-velopment in order to run the so-called solar-gas turbines (SGT). Solar ther-mal energy is used to heat compressed air to around 1000oC. Thus, natural gas consumption can be reduced. High solar-to-electricity efficiency levels are to be obtained due to high operating temperature of solar heat and in-trinsic efficiency of combined cycles. In addition, water consumption can be significantly reduced once Bryton gas cycles are used (QUERO et al., 2014).
Steam generators based on coal or biomass combustion can also be hy-bridized. This concept is here defined as hybridized steam generators (HSG). Peterseim et al. (2014a) and Nixon et al. (2012) evaluated the operation of CSP in parallel with conventional steam generators producing superheated steam with same parameters. Peterseim et al. (2014b) evaluated the effici-ency gain due to post superheating of steam produced with a parabolic trough solar field with biomass firing as the concept implemented in SHAMS power plant in United Arab Emirates. Zhao (2012) proposed the production of sa-turated steam to partially displace the load of a coal fired boiler. Produced saturated steam in solar field was injected back into the conventional steam generator drum for post superheating.
The most explored scheme in literature related to CSP hybridization with Rankine cycles is the so-called solar aided feedwater heating concept (SAFWH). It can be accomplished by the substitution of turbine bleed-off steam extractions by solar heat. Several works demonstrated that the higher the temperature of feedwater heater displaced, the higher the efficiency in terms of power boost or fuel economy (YANG et al., 2011; HOU et al., 2011;
POPOV, 2011;YAN et al., 2010;SURESH et al., 2010). As it is discussed by Zhao et al.(2014b) and Zhao et al. (2014a), solar-to-electricity efficiencies higher than the obtained on solar-only plants under similar operating temperature can be found as the exergy destruction associated to feedwater heating using bleed-off steam is avoided. As further related works, Bakos and Tsechelidou (2013) showed the economic advantages of using the existing infrastructure of a coal power plant to host a solar integration in a feedwater heating scheme in Greece. Hong-juan et al. (2013) studied the performance of a solar aided coal power plant under different loads and an optimization procedure was used to identify the most feasible solar multiple. Pierce et al. (2013) performed the comparison of a conventional solar-only parabolic trough plant with an
equivalent solar field size used for feedwater heating of a coal fired plant in South Africa. Finally, Peng et al. (2014) performed detailed analysis on off-design operation of a solar aided coal fired plant, pointing out the ways to improve system’s efficiency.
Relevant results presented in literature are summarized in Table 2 in order to drive a comparison in terms of important performance parameters. In case of adding solar energy to a conventional power plant, the solar share will be limited by the physical restrictions in existing equipments. The annual solar share of 2.4 % was reached in the ISCC concept presented by Bakos and Parsa (2013). In this work, the steam turbine of combined cycle was desig-ned to a higher capacity to improve solar participation in power output. In all SAFWH examples the annual solar share was limited to around 1 % of total electricity output. The improvement of this parameter requires a new project conceptually provided for hybrid operation. This was the case of power plant layout discussed by Peterseim et al. (2014a), where both solar field and bio-mass steam generators were designed to provide the same electricity output of 10 MW. In an annual basis, solar was able to contribute with 20.7 % of total electricity output. The commercial power plant Borges Termosolar lo-cated in Spain has the same conceptual design. Solar and biomass systems were balanced to obtain yearly 50 % solar participation on electricity output - see next section about commercial hybrid plants.
The peak solar-to-electricity efficiency of hybridized layouts ranged from 21 to 32 % depending on integration layout, solar field model, operation temperature and efficiency of components. Presented values were in most ca-ses higher than the inherent peak efficiency of the conventional CSP systems exposed in Table 1 of Section 2.1.2. Specifically in SAFWH cases, the solar field temperature was reduced to below the typical level of 400oC of
parabo-lic trough plants. The higher the solar-to-electricity efficiency, the smaller the size of solar field (and CAPEX, as a consequence) to generate certain amount of electricity.
Regarding economics, the presented cases indicated levelized cost of solar electricity ranging from the very low value of 70 up to 210 U$/MWh. These values are well below the results indicated for conventional CSP plants in Figure 5. Nevertheless, the comparison should be made cautiously since the input economic parameters are not uniform among all references.
T able 2: Literature data related to CSP h ybridization selected for cross-comparison. Publication CSP Output [MW] Layout T emperature o[C] ‡ Solar share [%] ♦ Location Peak ηse [%] LCOE [U$/MWh] ] ( B AK OS; P ARSA , 2013) T rough 50 C C-N A S ISCC 390 0.53-4.1 South Greece -( MONTES et al. , 2011) T rough 220 C C-N A S ISCC 300 2.4 Las V eg as, USA 27.3 -Peterseim et al. (2014a) T o wer 10 B-10 S HSG > 540 20.7 Mildura, A US -Peterseim et al. (2014b) T rough N A B-50 S HSG 390 -Mildura, A US 27.5 -Y ang et al. (2011) T rough 200 C-19.5 S SAFWH 260 -25.6 4 -Hou et al. (2011) T rough 12 C-0.4 S SAFWH 230 1.0 Lhasa, China 22 .6 77-17 0 ♣ Popo v (2011) Fresnel 117 O-27 S SAFWH 320 -Cyprus 27.3 4 -Y an et al. (2010) -200 C-20 S SAFWH 330 -32.1 4 -Suresh et al. (2010) T rough 660 C-25 S SAFWH 280 -India 23 .0 -Zhao et al. (2014a) T rough 1000 C-N A S SAFWH 300 -27.9 -Zhao et al. (2014b) T rough 200 C-20 S SAFWH 267 -Shizuishan, China 21.3 -Bak os and Tsechelidou (2013) T rough 278 C-24 S SAFWH 400 -Ptolemais, Greece 25.0 100 Hong-juan et al. (2013) T rough 307 C-50 S SAFWH 285 -Lhasa, China 26.2 7 1-152 ♣ Pierce et al. (2013) T rough 600 C-22 S SAFWH 350 0.9 Lephalale, S. Africa -Peng et al. (2014) T roug h 330 C-16 S SAFWH 300 -Sinkiang, China 26 .3 -Schuhmacher et al. (2013) Fresnel 12 B-0.5 S SAFWH 220 0.63 Brazil, MS 22.2 100-210 S: solar; C: Coal; B: Bi omass; O: Oil; CC: Combined cycle; GT : Gas T urbine; ‡Solar field maximum temperature; ♦Annual basis solar participation; ]Mar ginal LCOE related to solar equi v alent electricity; 4Estimated based on 70 % peak solar field thermal ef ficienc y; ♣1 Chinese yuan = 0.15 U$; 1 Euro = 1.30 U$.
2.1.6 Projects around the world
A list of hybrid CSP projects around the world is presented in Table 3. As it can be observed, most cases are related to the integration of solar energy into combined cycles - the so called ISCC power plants. All cases related to the integration of solar energy into coal fired power plants are based on SAFWH. Finally, the Borges power plant can be mentioned as the only one related to the hybridization of CSP with biomass. The parabolic trough so-lar field and biomass steam generators are operated in parallel to produce superheated steam with the same temperature and pressure parameters. The solar field was designed to operate the 22.5 MW steam turbine at 100 % load in design point DNI. Biomass system, on the other hand, was designed to operate the steam turbine at 50 % load. In an yearly basis, the equilibrium of 50 % solar share might be achieved according to the local weather conditions.
Table 3: Hybrid CSP projects around the world.
CSP Plant name Output [MW] Location Layout Start
Fresnel Liddell 2000C-9S Australia SAFWH 2008
Fresnel Kogan Kreek 750C-44S Australia SAFWH UC
Fresnel Mejillones 150C-5S Chile SAFWH PL
Trough Cameo 49C-1S USA SAFWH 2010
Trough ISCC Hassi 130CC-25S Algeria ISCC 2011
Trough Medicine Hat 203CC-1S Canada ISCC UC
Trough ISCC Kuraymat 120CC-20S Egypt ISCC 2011
Trough Yazd Solar 467CC-17S Iran ISCC 2009
Trough Archimede 760CC-5S Italy ISCC 2010
Trough Agua Prieta II 464CC-14S Mexico ISCC UC
Trough Beni Mathar 450CC-20S Morocco ISCC 2010
Trough Martin next 1150CC-75S USA ISCC 2010
Trough Palmdale 570CC-50S USA ISCC PL
Tower Karaman 450CC-50S Turkey ISCC UC
Tower Solugas 4.5GT-NAS Spain SGT 2012
Trough Borges 12.0B-22.5S Spain HSG 2012
S: Solar; C: Coal; B: Biomass; CC: Combined cycle; GT: Gas Turbine; UC: Under Construction; PL: Planned; Source: (SOLARPACES, 2015).
2.2 THE BRAZILIAN SUGARCANE SECTOR
2.2.1 Sugarcane sector figures
The land area used for sugarcane plantation in Brazil as well as the sugarcane crushed amount along the last nine harvest periods (plus estima-tion for the next 2014/15 harvest) are presented in Figure 6. Today the land area used for sugarcane plantation is around 9.0 Mha. It is the third crop in Brazil in terms of used land behind soybeans (28 Mha) and corn (16 Mha) plantations (COMPANHIA NACIONAL DE ABASTECIMENTO (CONAB), 2015). In Brazil, around 60 Mha are used for agricultural purposes, what corresponds to 7 % of national land area (REDEAGRO, 2010). The sugarcane plantations alone corresponds to 1.1 % of national land area.
Figure 6: Sugarcane plantation area in Brazil and annual crushing evolution.
Source: (COMPANHIA NACIONAL DE ABASTECIMENTO (CONAB), 2015).
The sugar and alcohol production along the last nine harvest periods (plus estimation for the next 2014/15 harvest) are presented in Figure 7. Most mills are able to produce both sugar and alcohol and the share is defined according to sugar and alcohol prices. It can be observed that in seasons 2011/12 and 2012/13 the production of sugar was prioritized due to the low price of alcohol in the market.
The production of sugarcane in Brazil is concentrated in the Center-South and North-Northeast regions. The distribution of crushed sugarcane in these two regions is presented in Figure 8 according to the annual milling capacity of factories. The results are based on the 2010/11 harvest period, when 623.9 Mt of sugarcane was crushed. As it can be observed, the capacity
Figure 7: Evolution of sugar and alcohol (total) production in Brazil.
Source: (COMPANHIA NACIONAL DE ABASTECIMENTO (CONAB), 2015).
of industries in the North-Northeast region is concentrated in small unities of less then 1.0 and 1.0-1.5 Mt/harvest. Plants are significantly bigger in terms of annual crushing capacity in the Center-South region of Brazil. Most cases are 2.0-3.0 Mt/harvest. Another important aspect related to both regions consists on the harvest period in these locations. While the harvest period is concentrated between April to December in the Center-South region, in North-Northeast it occurs between August to April (BNDES; CGEE, 2008).
Figure 8: Crushed sugarcane according to industries capacity.
2.2.2 Cogeneration cycles
The sugar and alcohol production demands thermal, mechanical and electrical energy. In this regard, the sugarcane mills are equipped with coge-neration power plants based on the combustion of bagasse - a residue obtained after the juice extraction process out of sugarcane culms. The elemental, pro-ximate and calorimetric properties of bagasse are presented in Table 4. The properties of sugarcane straw are also presented once its use has been evalu-ated recently as a way to complement bagasse in steam generators operating under co-firing3. The reported values are typical in the sugarcane sector and
represent the average of several evaluations (LAMˆoNICA; LINERO, 2013). The 50 % moisture content of bagasse consists on its humidity after the juice extraction process as it is feeded in steam generators - typically no drying process is implemented. The 15 % moisture content of straw consists on the average humidity of straw baled in the field (RODRIGUES, 2005).
Table 4: Elemental, proximate and calorimetric (heating value) analysis of sugarcane bagasse and straw.
Fuel type Bagasse Straw
Elemental analysis (dry, ash free [%])
Carbon 45.6 47.9 Hydrogen 5.8 6.4 Nitrogen 0.4 0.6 Oxygen 48.2 44.7 Sulphur 0 0.1 Chlorine 0 0.2
Proximate analysis (as received [%])
Ash 1.6 7.7
Fixed Carbon 6.9 13.6
Volatile matter 41.6 63.9
Moisture 50 15
Heating value (as received [kJ/kg])
Higher Heat Value (HHV) 9000 14450
Lower Heat Value (LHV) 7162 12996
Source: (LAMˆoNICA; LINERO, 2013).
3Energetically, for each ton of sugarcane culms there are around 150 kg of sugar (2,400 MJ) and 130 kg of dry bagasse (2,300 MJ). Additional 140 kg of dry straw (2,500 MJ) is left on the field during harvest process (RODRIGUES, 2005). Today part of straw potential is used in some demonstration projects in order to complement bagasse in steam generators (LEAL et al., 2013).
The thermal energy demand in sugarcane mills is related to the eva-poration of sugarcane juice and alcohol distillation. In conventional factories able to produce both sugar and alcohol, it is typically demanded 500 ton-nes of saturated steam (x=1; 2.5 bar) per ton of processed sugarcane culms. In new plants this index has been reduced to 300-350 tonnes of saturated process steam per ton of sugarcane culms due to optimized process thermal integration and minimization of thermal losses (ENSINAS, 2008). The mecha-nical energy necessary for sugarcane crushing and juice extraction is typically around 16 kWh per ton of sugarcane culms, while the electricity demand ne-cessary for motors, pumps, illumination, among other services, is typically around 12 kWh per ton of sugarcane culms (SEABRA, 2008).
The four main layouts of cogeneration plants applied to the sugarcane sector are presented in Figure 9. The layouts and operational parameters were optimized along the years, as it is discussed below, in order to improve the electricity production efficiency.
Figure 9: Layouts of cogeneration cycles applied to the sugarcane sector.
Sources: (SEABRA, 2008;NETO; RAMON, 2002).
Initially, the cogeneration plants were based on the use of back-pressure steam turbines (BPST) with exhaust steam back-pressure of 2.5 bar used to feed process thermal load. The superheated steam parameters of 22 bar and 300 oC, associated with the process thermal energy consumption of
500 tonnes of saturated steam (x=1; 2.5 bar) per ton of sugarcane culms, turned possible the self-production of mechanical and electricity demands while burning all the available bagasse produced in juice extraction process. The operation of these plants is dependent on the process operation. Thus, if process is off due to no sugarcane availability, power plant might also be turned off even if bagasse is stored. Furthermore, the reduction of process steam consumption implies bagasse surplus at the end of the harvesting period (NETO; RAMON, 2002).
Since 2000 the electricity exportation to the grid has been an important additional product to the sugarcane factories in Brazil and this was provided by the decentralization of national electricity sector. In this regard, the im-provement of cogeneration plants efficiency has been performed in different ways. One option was the retrofit of conventional cogeneration plants based on BPST turbines by replacing existing boilers by new unities with higher steam parameters. The efficiency improvement was so provided by the higher enthalpy drop of steam up to exhaust condition of 2.5 bar (SEABRA, 2008).
The implementation of condensing-extraction steam turbines (CEST) turned possible the operation of cogeneration plants independently on the pro-cess operation. In addition, it became convenient to minimize the consump-tion of process steam (x=1; 2.5 bar) once part of produced steam in boilers can be expanded up to condenser pressure. The efficiency of cogeneration plants was further improved by the substitution of the mechanical drivers in sugarcane mills by the electrified systems. The steam mechanical drivers were single stage low efficiency turbines (SEABRA, 2008).
The evolution of superheated steam parameters (maximum levels avai-lable in local industry) of steam generators produced by the main suppliers to the sugarcane sector is presented in Figure 10. From the 70s to 2000 the most common configuration was based on 22 bar and 300oC despite the
availa-bility of higher steam parameters in the market. Today the standard consists on steam generators able to produce steam at 67 bar and 520oC, although 120 bar and 520oC steam is also possible in new bubbling fluidized bed sys-tems.
The installed capacity of sugarcane bagasse cogeneration plants in Brazil reached 9,933.6 MW in the first semester of 2015. This amount is produced by 387 units and represents 6.9 % of the Brazilian electricity ins-talled capacity (ANEEL, 2015). In 2013, a total of 15,067 GWh of electricity generated by the sugarcane bagasse cogeneration plants was exported to in-dependent consumers supplying around 8 million homes. Since 2005, an average yearly growth in the electricity exportation of 34 % was observed
Figure 10: Evolution of main steam parameters in the sugarcane sector.
Sources: http://caldema.com.br/; http://www.equipalcool.com.br/; http://www.sermatec.com.br/; http://www.codistil.com.br/. Figure 11: Evolution exported electricity to the grid.
Source: (SOUZA, 2014).
(SOUZA, 2014) - see Figure 11.
2.2.3 The identified potential
The location of sugarcane cogeneration plants and the DNI incidence in Brazil is presented in Figure 12. As it can be observed, Brazil has a great potential regarding solar and biomass availability for electricity power gene-ration. In addition, both energy sources matches regionally in their availa-bility quite good. The DNI incidence can reach up to 2000 kWh/m2-year in
the Center-South region where most sugarcane mills are located - what is si-milar to the 2100 kWh/m2-year level found in South Spain. In this regard, it was identified in Brazil an opportunity related to the integration of solar thermal energy with the cogeneration plants of sugarcane sector. The solar boost might minimize the seasonality effect inherent to this crop extending the operation of the unities to the off-season period. As it is described in Chapter 3, the hybridization layouts evaluated in this work are based on the retrofit of a typical cogeneration power plant applied to the sugarcane sector with parabolic trough concentrators.
Figure 12: Identified potential of CSP hybridization in the sugarcane sector: a) bagasse power plants location; b) integrated annual DNI potential in Brazil.
3 HYBRID CONCEPT PROPOSAL
3.1 OPERATION STRATEGY
A schematic of a sugarcane processing industry assisted with solar thermal energy and the proposed operation strategy is presented in Figure 13. The cogeneration plant provides electricity and heat to the process where su-gar and alcohol are produced. Surplus electricity is exported to the grid. The concept proposed in this work consists on operating the cogeneration power plant in a fuel economy mode during harvest periods. Thus, stored bagasse is used to run the power plant during off-season.
Figure 13: Schematic of the process and concept illustration.
3.2 SELECTED INTEGRATION LAYOUTS
Several integration layouts based on distinct CSP technologies are pos-sible when performing the hybridization of conventional sugarcane bagasse cogeneration plants. This work is focused on parabolic trough technology applied for feedwater pre-heating and also saturated steam generation once it represents the most mature technology in terms of installed capacity ( SO-LARPACES - SOLAR POWER AND CHEMICAL ENERGY SYSTEMS, 2014). Both the
cogeneration cycle and steam generator layouts as well as the operational parameters were identified in cooperation with equipment suppliers of the su-garcane sector in Brazil. The feasibility of CSP hybridization was studied aiming minimal modifications on original plant.
The layouts studied in this work as well as additional identified pos-sibilities based on linear Fresnel and central tower are shown in Figure 14. All presented cases were evaluated in the research project New Partnerships (iNOPA) funded by the Coordination for the Improvement of Higher Edu-cation Personnel (CAPES) and by the German Academic Exchange Service (DAAD) and developed in cooperation between the Laboratory of Combus-tion and Thermal Systems Engineering (LabCET) of Federal University of Santa Catarina (UFSC) and the Chair of Environmental Process Engineering and Plant Design (LUAT) of University of Duisburg-Essen, Germany. Each CSP technology was applied according to the temperature level required by the integration layout. The results obtained with project execution are sum-marized in Appendix E and compared with results related to this work.
3.3 THERMOECONOMIC ANALYSIS
A set of performance indexes is described in this section in order to evaluate the integration of a base case cogeneration plant with a parabolic trough solar field. The annual thermal efficiency, ηs f [%], of the parabolic
trough solar field was calculated based on Equation 3.1,
ηs f = 100
106 ∑yearQ˙av
∑yearAs f Gbn
(3.1)
where ˙Qav [MW] is the net heat delivered by solar field, Gbn[W/m2] is the
direct normal irradiance and As f [m2] the solar field aperture area.
The economized amount of bagasse during harvest operating hours, EB[t], was calculated by Equation 3.2,
EB=
∑
harvest
[ ˙mb,bc− ˙mb,h] (3.2)
where ˙mb,bcand ˙mb,hare the bagasse consumption for base and hybrid cases
in tons per hour.
The additional power generated off-season, AE [MWh], due to the economized amount of bagasse was quantified by Equation 3.3,
AE=
∑
o f f−season
˙
Wh (3.3)
where ˙Wh[MW] is the net power output during off-season for the hybrid plant. Figure 14: Identified integration layouts of CSP and bagasse cogeneration cycles to be studied.
Taking additional power generated AE [MWh] into consideration, the annual solar-to-electricity efficiency was calculated by Equation 3.4.
ηse= 100
106AE ∑yearAs f Gbn
(3.4) The economic feasibility of a thermal system can be performed under
different methods. Examples of thermoeconomic analysis are found in Esen et al.(2006), Bhattacharjee and Dey (2014), Buonomano et al. (2015). In this work it was considered the Levelized Cost of Electricity (LCOE) [U$/MWh] calculation according to the methodology proposed in IEA (2010). The LCOE was calculated for the additional power generated off-season, AE [MWh], by comparing it to the capital and annual costs due to solar inte-gration, LCOE=∑ lt t=0(CC + LC + O&M) (1 + r)−t ∑ltt=0AE(1 + r)−t (3.5) where CC, LC and O&M [U$] are the capital, land and annual operation and maintenance costs. The parameter r represents the interest rate and lt [years] is the lifetime of plant.
4 SOLAR FIELD MODELING
4.1 MAIN COMPONENTS
A parabolic trough solar field is based on a set of loops which are re-plicated to reach the required thermal capacity. Each loop consists in a group of Solar Collector Assemblies (SCA) connected in series where the tempera-ture of the heat transfer fluid (HTF) is increased during operating hours. All loops are connected to main header pipes that supply the heat exchangers with hot HTF and sending back to the solar field the cooled HTF. A set of valves is used to provide recirculation at low solar radiation hours and at night time when the required HTF temperature is not reached. Recirculation is neces-sary during these periods to avoid thermal stress of solar field components. Yet, as HTF has considerable temperature changes, the volume increase due to its volumetric expansion is accommodated in an expansion tank and a set of overflow tanks. An example of a parabolic trough solar field of four loops, each one based on four SCA, is shown in Figure 151.
Figure 15: Schematic of a parabolic trough solar field.
A schematic of one of the parabolic trough loops shown in Figure 15
1The number of loops and the number of SCAs per loop may change depending on the specific project characteristics and SCA model. See in Chapter 7 that the proposed solar field layouts differ to the example presented in Figure 15.
is presented in detail in Figure 16. As it can be observed, each SCA has in-dependent tracking system and mirrors are rotated along its horizontal axis tracking the sun position. One SCA is composed by a set of Solar Collector Elements (SCE), which consists in a metallic structure that supports a parabo-lic shaped mirror and the Heat Collection Element (HCE) located in the focal line. The rows are spaced to minimize shadowing effects at the beginning and at the end of the days.
Figure 16: Schematic of a parabolic trough loop.
The HCE consists in a metallic tube coated on a selective surface with a high absorptivity for visible spectrum radiation and low emissivity for in-frared radiation. A glass envelope with high transmissivity is used to form a vacuum region around the metal tube receiver, thus minimizing losses by convection. A schematic of a HCE is shown at Figure 17.
Figure 17: Schematic of a HCE (not shown in scale).
Source: (BURKHOLDER; KUTSCHER, 2009).
In Figure 18 it is shown a solar field equipped with SkyTrough para-bolic solar collectors. It can be observed the SCE assemblies together with the HCE elements are positioned in the focal line. Each SCE has six meters
width, 13.9 m length and three HCEs of model SCHOTT PTR-80. One SCA has eight SCE assemblies, totalizing 115 m gross length, Lsca,g.
Figure 18: SkyTrough parabolic trough collectors.
Source: (SKYFUEL, 2011).
4.2 GEOGRAPHICAL AND METEOROLOGICAL DATA
The plant location might be defined considering the latitude angle φ [deg], the local longitude χloc[deg] and the reference meridian χre f [deg].
The latitude angle is negative to the south hemisphere and referenced to the equator. The local longitude is referenced to the Greenwich meridian and negative to west. Finally, the reference meridian is necessary to define the local time. There are in total 24 reference meridians of 15 degrees each. As an example, the state of Mato Grosso do Sul in Brazil is represented by -60o. When performing feasibility assessments of solar thermal systems by simulations it is necessary to consider representative meteorological data sets for the location of interest. A Typical Meteorological Year (TMY) summari-zes a long time series of measurements (e.g. 30 years) in order the typical we-ather conditions can be represented whereas the average effects of the whole data base is preserved. It is important to note that TMY data is not indicated to simulate extreme operational conditions, but typical. TMY data for some locations of Brazil can be found in SWERA database (SWERA, 2015). Impor-tant variables are solar radiation, ambient temperature and wind velocity.
